In olefin epoxidation, a reactor feed containing an olefin and oxygen is contacted with a catalyst under epoxidation conditions. The olefin is reacted with oxygen to form an olefin oxide. A product mix results that contains olefin oxide and, typically, unreacted reactor feed and combustion products.
Carbon dioxide is a by-product in the epoxidation process, and may be present in the reactor feed. Under commercial operation of epoxidation processes, the epoxidation reactor feed is formed by adding fresh oxygen and olefin to a recycle gas stream which comprises, besides unreacted and recycled oxygen and olefin, quantities of carbon dioxide, water, and other gases.
The olefin oxide may be reacted with water to form a 1,2-diol, with carbon dioxide to form a 1,2-carbonate, with an alcohol to form a 1,2-diol ether, or with an amine to form an alkanolamine. Thus, 1,2-diols, 1,2-carbonates, 1,2-diol ethers, and alkanolamines may be produced in a multi-step process initially comprising olefin epoxidation and then the conversion of the formed olefin oxide with water, carbon dioxide, an alcohol, or an amine.
The catalytic epoxidation of olefins using a silver-based catalyst has been known for a long time. Conventional silver-based epoxidation catalysts have provided the olefin oxides notoriously in a low selectivity. For example, when using conventional catalysts in the epoxidation of ethylene, the selectivity towards ethylene oxide, expressed as a fraction of the ethylene converted, does not reach values above the 6/7 or 85.7 mole-% limit. Therefore, this limit has long been considered to be the theoretically maximum selectivity of this reaction, based on the stoichiometry of the reaction equation7C2H4+6O2=>6C2H4O+2CO2+2H2O,cf. Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd ed., vol. 9, 1980, p. 445.
Modern silver-based catalysts however are more selective towards olefin oxide production. When using the modern catalysts in the epoxidation of ethylene, the selectivity towards ethylene oxide can reach values above the 6/7 or 85.7 mole-% limit referred to. Such highly selective epoxidation catalysts are known from U.S. Pat. No. 4,766,105 and U.S. Pat. No. 4,761,394. However, the highly selective epoxidation catalysts employ higher reaction temperatures than do the conventional epoxidation catalysts for a given ethylene oxide yield, and they exhibit a greater rate of catalyst deactivation than conventional epoxidation catalysts.
The selectivity is the fraction of the converted olefin yielding the desired olefin oxide. As the catalyst ages, the fraction of the olefin converted normally decreases with time and to maintain a constant level of olefin oxide production the temperature of the reaction may be increased. However, this temperature increase adversely affects the selectivity of the conversion to the desired olefin oxide.
The selectivity determines to a large extent the economical attractiveness of an epoxidation process. For example, one percent improvement in the selectivity of the epoxidation process can substantially reduce the yearly operating costs of a large scale ethylene oxide plant. Further, the longer the activity and selectivity can be maintained at acceptable values, the longer the catalyst charge can be kept in the reactor and the more product is obtained. Quite modest improvements in the selectivity, activity, and maintenance of the selectivity and activity over long periods yield substantial dividends in terms of process efficiency.
International Patent Application WO 2004/078737 discusses the improvement in performance of highly selective epoxidation catalysts during the production of ethylene oxide when the reactor feed contains less than 2 mole-% of carbon dioxide, relative to the total reactor feed.
It is desirable to find a way to further improve the epoxidation process, for example improving the selectivity of a highly selective epoxidation catalyst in the manufacture of olefin oxide while also improving the stability of such catalyst.